In the general field of oil well pumping, it is usual to employ a vertically reciprocating sucker rod string inside the well casing, the purpose of which is to continuously operate a pumping mechanism at the bottom of the well which continuously pumps the oil up through the well tubing or casing. The sucker rod string is an assembly of sucker rods joined end-to-end in a straight line by a plurality of couplings, one coupling between each pair of vertically adjacent sucker rods. Usually, the ends of the adjacent sucker rods are externally threaded, and the coupling joining two adjacent sucker rods is internally threaded so that it can receive the ends of the adjacent sucker rods. Because of the cyclical stressing of every sucker rod string joint during the continuous operation of the pump, certain weak points in the string, particularly the threads which can be locations of stress concentration, are subject to metal fatigue failures. In the past, considerable design work has been done on a form of sucker rod pin end and coupling which would minimize the sensitivity of the total assembly to metal fatigue.
An added complication in many oil wells is the presence of corrosive materials pumped up along with the oil. Such wells, often referred to as "sour" wells because of the presence of certain corrosive compounds present a particular problem to the design of the sucker rod string because of the tendency for the corrosive materials, which can be liquid or gaseous, to find their way into the space between the interlocked threads of a coupling and a sucker rod. Because the threads are locations of high stress concentration to begin with owing to their shape, and because high tensional forces are transmitted through the engaged threads during the operation of the well, the corrosive materials tend to attack the threads and cause deterioration, weakness and sometimes failure of the joint.
One conventional joint between a coupling and a sucker rod end includes a surface-against-surface metal contact which, if properly made up and sufficiently torqued prior to service, is able at least initially to prevent the corrosive materials from penetrating into the location where the threads are in engagement. Theoretically, the proper torquing of these joints should prevent the joint faces from separating during service, and the threads should thus be shielded from any corrosive well environment. However, it often happens that the joint is not correctly made up, or is not properly torqued, with the result that the surface-against-surface contact can separate during prolonged service, thus permitting the corrosive well medium to seep into the joint assembly cavity and attack the threads.
It will be appreciated that, even if the parts are properly made up and properly torqued prior to service, a separation of the metal-against-metal seal can result from a partial unscrewing of the threaded portions, or from a gradual loosening of the threads during the operation due to "cold working" effects and to simple mechanical wear.
Present-day techniques for counteracting the tendency of the threads to become looser include the provision of viscous sealing compound completely filling up the contact area. This method has not been entirely satisfactory, however, because if the sealing compound hardens, it may have a tendency to crack and thus break the seal, whereas if the sealing compound remains viscous, it could be displaced by entering gases which thus would gain access to the parts intended to be shielded.
Another conventional method of protecting the engaged threads from attack by a corrosive well environment is to apply a particular coating material to the threads so as to prevent access to the threads by any corrosive substance. In this connection, reference may be had to U.S. Pat. No. 2,940,787, entitled "Electrically Insulated Sucker Rod Coupling", and issued June 14, 1960 to R.V. Goodner. This patent discloses the application of a protective coating to the threads, the coating being initially in liquid form to permit it to spread uniformly and in a thin layer over the thread surfaces. Prior to coating, the threads themselves are given rounded edges. When the coating has set, hardened or solidified, it then constitutes a form of protection for the threads. The shortcoming of this solution to the problem of thread corrosion is simply that it is difficult to totally ensure against rupture or scraping off of the coating during the torquing operation. A further drawback is related to the tendency of the sucker rod string to bend or undulate to some extent during operation. This action will cause differential pressure at different locations of the engaged threads whereby the thread contact at one portion of the periphery of the sucker rod may be similar to that resulting from an urging together of the coupling and the sucker rod, while the engagement of the threads at a point 180.degree. away from the first location is just the opposite. This action tends to concentrate stresses at particular locations around the thread which, in addition to the natural stress-concentration characteristics of the thread profile, greatly add to the danger of rupturing any such coating as is proposed in U.S. Pat. No. 2,940,787.